This invention relates to a thermosetting resin composition for injection molding and an article having a body formed by injection molding of the resin composition and a metal coating film formed by vacuum deposition.
Both thermoplastic resins and thermosetting resins have been used as molding materials for producing various articles. However, the resins of both types are inferior in heat resistance to metal materials, and this shortcoming has limited the application of the resin materials. More particularly, thermoplastic resins used as molding materials are mostly below about 180.degree. C. in heat distortion temperature, and even certain thermoplastic resins of which the heat distortion temperature is above 180.degree. C. are not higher than 180.degree. C. in glass transition temperature and, therefore, undergo some changes in the dimensions of the molded articles when long used at temperatures higher than the glass transition temperature. Some popular thermosetting resins such as phenolic resins and unsaturated polyester resins are 190.degree.-200.degree. C. or above in heat distortion temperature, but, neverthless, moldings of these thermosetting resins cannot said to be chemically stable at temperatures up to the heat distortion temperature because the moldings liberate some gases even when kept at relatively lower temperatures such as 120.degree.-150.degree. C.
The liberation of gases from the moldings becomes a serious problem particularly when the moldings are provided with coating films formed by painting or vacuum deposition, because the gases liberated from the heated moldings produce pressures at the interface between the molded resin body and the coating film and often cause blistering or even peeling of the coating film. For example, when a lamp reflector experimentally produced by coating a molded body of a novolak-type phenolic resin base composition with a base coat of a resin, a metal film formed by vacuum deposition and a top coat of a resin was kept at about 150.degree. C., significant blistering of the metal film and the top coat was observed within one hour. Similar blistering phenomena were observed also when resol-type phenolic resin and unsaturated polyester resin were individually used as the material of the reflector body. Additionally it was confirmed that post curing of the molded reflector body prior to the application of the base coat had the effect of somewhat raising the temperature at which the subsequently deposited metal film blistered within one hour. However, even in that case the blistering occurred within one hour at temperatures below the heat distortion temperatures of the employed thermosetting resins, and from an industrial point of view the post curing of the molded bodies is a time- and energy-consuming treatment which is desired to be omitted in order to reduce the production cost. Also it was confirmed that the blistering temperature could be rendered somewhat higher by raising the mold temperature and/or prolonging the curing time in the injection molding operation, though unfavorable for efficiency of the operation.
From these experimentally confirmed facts, a primary reason for the blistering phenomenon is presumed to be incompleteness of the curing reaction of the thermosetting resin, i.e. condensation polymerization reaction in the case of a phenolic resin, subjected to injection molding to result in the existence of unreacted organic materials in the moldings. When the moldings or articles produced by using the moldings are kept at high temperatures the unreacted materials will undergo polymerization reaction with formation of some gaseous substances. In an experiment with respect to novolak-type phenolic resin which is one of the most popular modling materials and is cured usually by using hexamethylenetetramine as curing agent, the liberation of ammonia gas from a heated molding was so significant that blow-holes produced in a base coat layer on the molding were clearly perceptible with the naked eye. Besides the gases formed by condensation polymerization reaction, moisture and/or certain organic volatile substances existing in the molded articles are considered to turn into gases that cause the blistering because the blistering phenomenon occurs even when the molding material is unsaturated polyester resin that cures by addition polymerization reaction.
Meanwhile, in the automobile industry it has been tried to produce reflectors for lamp units such as head lamp units by injection molding of a thermosetting resin firstly because at present the freedom of designing the shape of the lamp reflectors are restricted by the limitations to the deep drawing for shaping the conventionally used sheet metals and secondly it is desired to reduce the weight of each reflector for the purpose of reducing the gross weight of the vehicle. However, relatively low heat resistance of thermosetting resins compared with metals has offered serious problems to the development of the resin base reflectors since the reflectors are required to be high in thermal stability.
In automobile head lamp units, for example, the surface temperature of the reflector often becomes above 150.degree. C. and reaches 200.degree. C. or higher in some areas because of a relatively small volume of the space in each lamp unit relative to the power of the lamp and, in some cases, also because of a short distance between the lamp and the reflector surface. Therefore, the liberation of considerable amounts of gases from the molded reflector body during operation of the lamp becomes a serious disadvantage. The insufficiency of the thermal stability of the molded reflector body leads to not only destruction of the interlaminar adhesion of the coating films but also to significant lowering in the reflectivity of the reflector surface and lowering in the mechanical strength of the reflector body itself.
As mentioned hereinbefore, it is unfavorable for industrial production of the resin base reflectors to perform post curing of the molded reflector bodies to thereby enhance the thermal stability of the reflector bodies. As a different way of obtaining resin base reflectors in which the blistering temperature becomes about 180.degree. C. or higher and the blistering becomes relatively small in scale, it has been tried to use a certain unsaturated polyester base molding compound of wet premix type, particularly a so-called bulk molding compound of low shrinkage class, as the molding material for the reflector bodies. However, wet premix compounds of this class are relatively high in specific gravity and, as a more serious disadvantage, are inferior in workability or moldability in injection molding processes and difficult to mold into intricately shaped bodies that are high in the surface precision.